Circadian rhythms are self-sustaining, 24-hour cycles in molecular, biochemical, and behavioral parameters that help an organism prepare for anticipated changes in physiological demand. Many important cardiovascular factors, including metabolism, heart rate, blood pressure, and hormone release, oscillate over a 24-hour period. In humans, the incidence of adverse cardiac events, such as myocardial infarction, ventricular tachycardia, and death from ischemic heart disease, vary according to the time of day, and forced changes in circadian rhythm are associated with increased risk for heart failure. Cardiovascular disease is the major cause of death in the United States and its incidence is reaching epidemic proportions worldwide. Despite overwhelming evidence of the importance of circadian rhythms in cardiovascular health, little is known regarding the circadian regulation of intracellular signaling pathways controlling cardiac function and remodeling. We have recently found evidence of large circadian oscillations in the activity of the calcium-activated protein phosphatase calcineurin in normal, healthy hearts. This finding is remarkable because activation of calcineurin has primarily been thought of as a pathological process driving cardiac hypertrophy and failure. We hypothesize that daily oscillations in calcineurin activity form interdependent feedback loops with other cellular processes helping to coordinate changes in cardiac function and remodeling in anticipation of changes in physiological demand. Furthermore, we postulate that disruption of the normal temporal relationship of calcineurin activity increases cardiac stress and contributes to deterioration of cardiac function. The goal of this grant is to identify the cause of circadian changes in calcineurin activity and to determine the role of calcineurin-dependent oscillations in cardiac health and disease. Specific Aim 1: To identify factors underlying the circadian rhythm in calcineurin activity. We will test whether extrinsic factors, such as physical activity, or intrinsic factors, such as cardiomyocyte- autonomous calcium oscillations are the primary cause underlying circadian activation of calcineurin. Specific Aim 2: To determine mechanisms through which circadian changes in calcineurin- dependent activities impact cardiac function. We will examine both changes in phosphorylation of regulatory proteins and direct transcriptional targets of the calcineurin/NFAT pathway. Specific Aim 3: To test whether disruption of normal circadian rhythms promotes pathological remodeling of the heart. Altered light regimens and transgenic lines with altered calcineurin activity will be used to disrupt normal circadian rhythmicity. Changes in cardiac function and survival when the animals are subjected to pressure overload will be assed as well as changes in the molecular mechanisms explored in Aims 1 and 2. (End of Abstract)